US 7359444 B1 Abstract An inverse discrete Fourier transform (IDFT) processor processing a lower frequency image of a selected transmit band along with base images of remaining transmit bands called for in a corresponding X-DSL standard. A hybrid upconverter processing the IDFT output by generating a higher frequency replication of the imaged transmit band falling in the standards based frequency range along with the base band images of remaining transmit bands and selectively passing onto the communication medium only images or replications called for by the standards. A hybrid downconverter processing at least one of the received bands at sampling rates below Nyquist thereby creating a lower frequency image within the range of a discrete Fourier transform (DFT) processor. The hybrid downconverter clears the frequency band in which the image processed at sub-Nyquist rates will fold. The DFT then transforms only the images or replications falling within its frequency range.
Claims(1) selecting for at least one relatively higher frequency transmit band, a spectrally compatible lower frequency gap in a transmit portion of the band plan;
performing a frequency-to-time transform of a lower frequency image of the selected transmit band concurrently with frequency-to-time transforms of any remaining non-selected transmit bands;
upconverting time domain resultants to generate both base band images as well as replications thereof with an amount of the upconversion required determined by a requirement that the transformed image of the selected transmit band overlap with the selected transmit band on the communication medium; and
filtering out the undesired images, either baseband or higher frequency replications thereof and passing remaining mix onto the communication medium.
Description This application claims the benefit of prior filed now abandoned Provisional Application No. 60/553,655 filed on Mar. 15, 2004 entitled “Extending the effective frequency range of FFT and IFFT in DMT modems through the use of folded images” which is incorporated herein by reference in its entirety as if fully set forth herein. 1. Field of Invention The present invention is generally related to modems, and more particularly to modems supporting multi-tone multi-band modulation protocols. 2. Description of the Related Art Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. An X-DSL modem operates at frequencies higher than the voiceband frequencies, thus an X-DSL modem may operate simultaneously with a voiceband modem or a telephone conversation. Currently there are over ten discrete X-DSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. XDSL modems are typically installed in pairs, with one of the modems installed in a home and the other in the telephone companies central office (CO) switching office servicing that home. This provides a direct dedicated connection to the home from a line card at the central office on which the modem is implemented through the subscriber line or local loop. What is needed is scalable modem architecture that allows a more flexible hardware response to the evolving X-DSL standards and the problems associated with providing hardware to handle each new standard. A method and apparatus is disclosed for extending the frequency range of multi-tone modems without the requirement of an increase in the size, frequency range or complexity of the associated Fourier transform circuit or processes. The approach is of particular advantage with respect to X-DSL protocols calling for multiple frequency division multiplexed upsteam and downstream bands. Signal generation is accomplished by allowing a Inverse Discrete Fourier Transform (IDFT) processor to process a lower frequency image of at least one selected transmit band along with such other base images of any remaining lower frequency transmit bands as may be called for in a corresponding X-DSL standard. A hybrid upconverter then processes the time domain output of the DFT by generating a higher frequency replication of the imaged transmit band which falls in the required standard based frequency range along with the baseband images of all remaining transmit bands and replications thereof. The hybrid upconverter selectively passes onto the communication medium, e.g. subscriber line, only the mix of images and/or replications thereof as called for the corresponding X-DSL standard. In a typical multi-band standard with multiple discrete transmit bands, this results in processing of some of the transmit bands at sampling rates below the Nyquist rate and others at sampling rates at or above the Nyquist rate. Signal detection also supports processing a mix of receive bands at sampling rates above and below the Nyquist rate. A hybrid downconverter processes at least one of the received bands at sampling rates below Nyquist thereby creating a corresponding lower frequency image that preserves all information content albeit at a lower frequency band within the range of the discrete Fourier transform (DFT) processor. The hybrid downconverter first clears the frequency band in which the image of the receive band(s) processes at sub-Nyquist rates is going to fold. The DFT then effects time-to-frequency domain transformations only on the mix of images and/or lower frequency replications thereof as fall within its frequency range. Due to the operation of the hybrid downconverter this mix includes all the information on the received communication channel. These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: A method and apparatus is disclosed for extending the frequency range of multi-tone modems without the requirement of an increase in the size, frequency range or complexity of the associated Fourier transform circuit or processes. The approach is of particular advantage with respect to X-DSL protocols calling for multiple frequency division multiplexed upsteam and downstream bands. The maximum frequency range specified for a particular X-DSL band plan, and the tone spacing, dictate the size of discrete Fourier transform (DFT) and inverse discrete Fourier transform (IDFT) design in discrete multi-tone (DMT) modems. Also, in practical implementations the DFT and IDFT size, and hence complexity, grow in multiples of 2. For example one can have 1024, 2048, or 4096 tone DFT/IDFT. As an example, for the International Telecommunications Union, ITU-T G.993.1 Bandplan-1 (same as American National Standards Institute Plan 998, ETSI bandplan), a.k.a. plan 998, with 4.3125 kHz tone spacing, specified in the standards, the maximum frequency range in the upstream direction is 12 MHz. This translates into a requirement of a 4096 tone FFT. However, it is worth noting that only a small fraction of these tones are actually used for transmission of data, and a lot of spare capacity is wasted. For this particular bandplan less than one third of the available spectrum is actually used. The DFT must have the capability to process signals up to 17.6 MHz even though only less than 5 MHz of that capacity is used. In an embodiment of the current invention a novel combination of reduced complexity Fourier transform engines coupled to corresponding hybrid up/downconverters allows folded and non-folded images to be used in transmitting and receiving a multi-band communication channel. By judicious choice of the sampling frequency, and by appropriate filtering of the original signal or the folded images, signal generation and detection is effected by DFT/IDFT engines, for signals that on the communication medium, extend beyond the frequency range of the DFT and/or DFT engines. This results in a more efficient utilization of the IDFT/DFT capacity. There are two parts to this invention. One is the generation of the signal at the transmit end by the IDFT, and the other is the detection of the signal at the receive end by the DFT. Signal Generation In the generation of the signal, the output of the IDFT already consists of the original signal and the higher frequency folded images, which are symmetrically placed around Fs/2, where Fs is the sampling frequency. Usually the sampling frequency would also correspond to the Nyquist frequency appropriate for the particular size of the IDFT. If the desired transmit frequency band at the output is beyond the IDFT frequency range (Fs/2), one can create and process a lower frequency image within the frequency range of the IDFT, such that after folding the replicated image falls in the frequency band of interest. Subsequently the original lower frequency image that is no longer needed will be filtered out, leaving only the frequency band of interest on the communication medium along with such other baseband images as are called for by the relevant X-DSL standard. Signal Detection For detecting signals that are beyond the DFT frequency range, the received signal is sampled first at a sub-Nyquist rate, thereby creating images that preserves all of the information content, but in a lower band within the range of the DFT, and then filter out the undesired frequencies. Before this sub-Nyquist sampling takes place the frequency band on which the image is going to fold onto has to be cleared of all unwanted signals through filtering. Generally in signals processing, one makes sure that the sampling frequency is equal or higher than the Nyquist rate to avoid aliasing. However, in this particular invention, aliasing is used to advantage to translate the received frequency bands to a lower range, where they can be processed. The opposing modem Communication device The opposing modem The reduction in DFT/IDFT size is the result of the mapping of the higher frequency one of the downstream bands The processing of the lower frequency band The hybrid encoder thus serves to normalize the higher frequency one of the downstream bands The processing of the lower frequency band The hybrid decoder A traditional approach would require a 4096 tone IFFT and FFT with 8.625 kHz tone spacing to process the signal in the upstream direction because the signal spectrum extends beyond 17.6 kHz. We will show how the technique explained in this invention is able to do it with a 2048 tone IFFT and FFT, and hence result in significant savings in complexity. At the receiver, the input signal has a spectrum similar to what is shown in The transmitter One implementation of the transmit section of this modem, using the method of this invention, is shown in As shown in The transmit and receive methods can be applied independently or jointly. The transmitter can be implemented in a traditional way and the receiver with the method described here. Conversely, the transmitter can use this method and the receiver can be a traditional one. Alternately, both the transmitter and receiver can use this method to reduce complexity. Of course, depending on which approach is selected, one would have to pay proper considerations to bit loading, tone ordering, and other associated algorithms. The filtering needed in this method can take place in analog or digital domain. This will also depend on the choice of the sampling frequencies in DAC and ADC in any particular implementations. Also the use of interpolation and decimation would be dependent on the above sampling rates. In these examples integer interpolation and decimation are used for simplicity. However, the method would be applicable for non-integer ratios of interpolation and decimation as well. In the above examples a single band was generated and detected that fell outside the IDFT and DFT frequency range. The same principles can be applied to generate and detect multiple bands as well, to make full use of the capacity of the IDFT and DFT. However, in these situations the complexity of the filtering will increase. Lastly, while the method was described in connection with a DMT modem for VDSL, it should be applicable in other applications requiring similar signal processing functions. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents. Patent Citations
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